Method and apparatus for transmitting and receiving data in wireless communication system

ABSTRACT

Methods and apparatuses are provided in a wireless communication system in which a radio resource control (RRC) message is received to change or release a secondary cell group (SCG). A re-establishment for a radio link control (RLC) entity of a terminal is performed based on the RRC message. A packet data convergence protocol (PDCP) data recovery is performed based on the RRC message. Performing the PDCP data recovery includes selectively transmitting a PDCP protocol data unit (PDU) previously submitted to the re-established RLC entity, for which a successful delivery has not been confirmed by the re-established RLC entity.

PRIORITY

This application is a Continuation Application of U.S. patentapplication Ser. No. 16/189,127, filed in the U.S. Patent and TrademarkOffice on Nov. 13, 2018, which is based on and claims priority under 35U.S.C. § 119 to Korean Patent Application Nos. 10-2017-0149796 and10-2018-0075786, filed on Nov. 10, 2017 and Jun. 29, 2018, respectively,in the Korean Intellectual Property Office, the disclosures of each ofwhich are incorporated by reference herein in their entireties.

BACKGROUND 1. Field

The disclosure relates generally to wireless communication systems, andmore particularly, to methods and apparatuses for transmitting andreceiving data for reducing a data transmission delay in a wirelesscommunication system.

2. Description of Related Art

To meet the increase in the demand for wireless data traffic after thecommercialization of Fourth generation (4G) communication systems,efforts has been made to develop an improved Fifth generation (5G)communication system or a pre-5G communication system, which are alsoreferred to as ‘beyond 4G network communication systems’ or ‘post longterm evolution (LTE) systems’.’ In order to achieve a high datatransmission rate, 5G communication systems are being developed to beimplemented in a band of extremely high frequency, or millimeter wave(mmWave), such as a 60 gigahertz (GHz) band. In order to reduce theoccurrence of stray electric waves in a band of extremely high frequencyand to increase the transmission distance of electric waves in 5Gcommunication systems, various technologies such as beamforming, massivemultiple input multiple output (MIMO), full dimensional MIMO (FD-MIMO),array antennas, analog beam-forming, and large scale antennas are beingactively researched.

In order to improve system networks for 5G communication systems,various technologies such as evolved small cell, advanced small cell,cloud radio access network (cloud RAN), ultra-dense network,device-to-device communication (D2D), wireless backhaul, moving network,cooperative communication, coordinated multi-points (CoMP), andinterference cancellation have been developed. Other technologies suchas hybrid modulation of frequency-shift keying (FSK) and quadratureamplitude modulation (QAM) (FQAM) and sliding window superpositioncoding (SWSC), which are advanced coding modulation (ACM) schemes, andfilter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA),sparse code multiple access (SCMA), have been developed for 5Gcommunication systems.

The Internet has evolved from a human-based connection network, wherehumans create and consume information, to the Internet of things (IoT),where distributed configurations, such as objects, exchange informationwith each other to process the information. More recent times have seenthe introduction of the Internet of everything (IoE) technology, inwhich technology related to the IoT is combined with technology forprocessing big data through connection with a cloud server. In order toimplement the IoT, various technical components are required, such as asensing technique, wired/wireless communication and networkinfrastructures, a service interfacing technique and a securitytechnique.

In recent years, techniques including a sensor network for connectingobjects, machine to machine (M2M) communication, and machine typecommunication (MTC) have been researched. In the IoT environment,intelligent Internet technology services may be provided to collect andanalyze data obtained from objects connected to each other, and thus, tocreate a new value for human life. As existing information technology(IT) techniques and various industries are converged and combinedtherebetween, the IoT may be applied to various fields, such as smarthomes, smart buildings, smart cities, smart cars or connected cars,smart grids, health care, smart home appliances, and high qualitymedical services.

Various attempts are being made to apply 5G communication systems to theIoT network. For example, various technologies related to sensornetworks, M2M communication, and MTC, are implemented by usingbeam-forming, MIMO, and an array antenna, for example. The applicationof the cloud RAN as a big data processing technique described above maybe an example of convergence of the 5G communication technology and theIoT technology.

In order to meet the increasing demand for large communication capacity,a number of techniques such as a method of providing a number ofconnections has been proposed. In an LTE system, a carrier aggregation(CA) technique may provide a number of connections by using a number ofcarriers, so that users may receive services via a number of resources.Also, various services including a multimedia broadcast multicastservice (MBMS) may be provided through the LTE system.

In the existing art, however, there is a shortcoming in thattransmission delay often occurs in the 5G communication technology.Thus, there is a need in the art for a method and apparatus thatmitigate such transmission delay.

SUMMARY

An aspect of the disclosure is to address at least the above-mentionedproblems and/or disadvantages and to provide at least the advantagesdescribed below. Accordingly, an aspect of the disclosure is to providemethods and apparatuses for transmitting and receiving data for reducinga data transmission delay.

Another aspect of the disclosure is to provide methods and apparatusesin which an equipment supporting a dual connectivity (DC) in a wirelesscommunication system further efficiently performs a procedure ofretransmitting data when a base station to which the equipment isconnected is changed.

In accordance with an aspect of the disclosure, a method, performed by aterminal in a wireless communication system is provided. A radioresource control (RRC) message is received to change or release asecondary cell group (SCG). A re-establishment for a radio link control(RLC) entity of the terminal is performed based on the RRC message. Apacket data convergence protocol (PDCP) data recovery is performed basedon the RRC message. Performing the PDCP data recovery includesselectively transmitting a PDCP protocol data unit (PDU) previouslysubmitted to the re-established RLC entity, for which a successfuldelivery has not been confirmed by the re-established RLC entity.

In accordance with another aspect of the disclosure, a terminal inwireless communication system is provide. The terminal includes atransceiver and at least one processor coupled with the transceiver. Theat least one processor is configured to receive an RRC message to changeor release an SCG, perform a re-establishment for an RLC entity of theterminal based on the RRC message, and perform a PDCP data recoverybased on the RRC message. The at least one processor is furtherconfigured to selectively transmit a PDCP PDU previously submitted tothe re-established RLC entity, for which a successful delivery has notbeen confirmed by the re-established RL entity.

In accordance with another aspect of the disclosure, a method performedby a base station in a wireless communication system is provided. An RRCmessage is transmitted to change or release an SCG. A re-establishmentfor an RLC entity of the terminal is performed based on the RRC message.A PDCP data recovery is performed based on the RRC message. A procedureof the PDCP data recovery includes selectively retransmitting a PDCP PDUpreviously submitted to the re-established RLC entity, for which asuccessful delivery has not been confirmed by the re-established RLCentity.

In accordance with another aspect of the disclosure, a base station in awireless communication system is provided. The base station includes atransceiver and at least one processor coupled with the transceiver. Theat least one processor is configured to transmit an RRC message tochange or release an SCG. A re-establishment for an RLC entity of theterminal is performed based on the RRC message. A PDCP data recovery isperformed based on the RRC message. A procedure of the PDCP datarecovery includes selectively retransmitting a PDP PDU previouslysubmitted to the re-established RLC entity, for which a successfuldelivery has not been confirmed by the re-established RLC entity.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the present disclosure will be more apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a structure of a long term evolution (LTE) system towhich embodiments of the present disclosure are applicable;

FIG. 2 illustrates a wireless protocol structure in an LTE system towhich embodiments of the present disclosure are applicable;

FIG. 3 illustrates a structure of a next generation mobile communicationsystem to which embodiments of the present disclosure are applicable;

FIG. 4 illustrates a wireless protocol structure of a next generationmobile communication system to which embodiments of the presentdisclosure are applicable;

FIG. 5 illustrates a structure for processing data in an LTE system,according to an embodiment;

FIG. 6 illustrates a structure for processing data in a next generationmobile communication system, according to an embodiment;

FIG. 7 illustrates a system to which a data transmission method of adual access UE is applicable, according to an embodiment;

FIG. 8 illustrates a procedure in which a UE to which a dual accesstechnology is applied receives a secondary cell group (SCG) changemessage or an SCG connection-release message, according to anembodiment;

FIG. 9 illustrates a procedure in which a UE detects a radio linkfailure (RLF) in an SCG and declares the RLF, according to anembodiment;

FIG. 10 illustrates a method of transmitting data when a UE receives anSCG change message or an SCG release message, according to anembodiment;

FIG. 11 illustrates a data transmission method of a UE, according to afirst embodiment;

FIG. 12 illustrates a data transmission method of a UE, according to asecond embodiment;

FIG. 13 illustrates a method of selectively retransmitting data when aUE detects an RLF, according to an embodiment:

FIG. 14 illustrates a method, performed by a UE, of sequentiallyretransmitting data of which reception is not acknowledged when an RLFis detected, according to an embodiment;

FIG. 15 is a block diagram of a UE according to an embodiment; and

FIG. 16 is a block diagram of a base station according to an embodiment.

DETAILED DESCRIPTION

Hereinafter, the present disclosure will be described in detail withreference to the accompanying drawings. Detailed descriptions of knownfunctions and configurations incorporated herein will be omitted when itmay make the subject matter of the present disclosure rather unclear.The following terms are defined in consideration of the functions of thepresent disclosure, and may be changed according to the intention of theuser, the operator, or custom, for example. Therefore, the definitionshould be based on the contents throughout the present specification.

In the following description, terms used for identifying an access node,referring to a network entity, referring to messages, indicating aninterface between network objects, and indicating various pieces ofidentification information, are provided for convenience of description.Therefore, the present disclosure is not limited to the following terms,and other terms referring to objects having equivalent technicalmeanings may be used.

For convenience of description, the present disclosure uses terms andnames defined in the 3rd generation partnership project long termevolution (3GPP LTE) standard. However, the present disclosure is notlimited by the above-mentioned terms and names, and may be equallyapplied to systems conforming to other standards.

FIG. 1 illustrates a structure of an LTE system to which embodiments ofthe present disclosure are applicable.

Referring to FIG. 1, a RAN of the LTE system may include evolved nodes B(eNBs, nodes B or base stations) 105, 110, 115, and 120, a mobilitymanagement entity (MME) 125, and a serving-gateway (S-GW) 130. A UE 135may access an external network through the eNBs 105 to 120 and the S-GW130.

In FIG. 1, the eNBs 105 to 120 may each correspond to the existing NodeB of the universal mobile telecommunications system (UMTS). At least oneof the dNBs 105 to 120 may be connected to the UE 135 via a wirelesschannel and may perform a more complex role than the existing Node B. Inthe LTE system, because all user traffic including a real-time servicesuch as voice over Internet protocol (VoIP) through the Internetprotocol is serviced through a shared channel, there is a need for adevice for collecting and scheduling status information such as a bufferstatus of UEs, an available transmission power status, and a channelstatus, which may be performed by the eNBs 105 to 120. One eNB maynormally control a plurality of cells. For example, to realize atransmission rate of 100 megabits per second (Mbps), the LTE system mayuse orthogonal frequency division multiplexing (OFDM) as a radio accesstechnique at a bandwidth of 20 megahertz (MHz) and may apply adaptivemodulation and coding (AMC) which determines a modulation scheme and achannel coding rate according to the channel status of the UE 135. TheS-GW 130 provides a data bearer and may generate or remove the databearer by the control of the MME 125, which performs various controlfunctions as well as a mobility management function with respect to theUE 135 and may be connected to a plurality of base stations.

FIG. 2 illustrates a wireless protocol structure in an LTE system towhich embodiments of the present disclosure are applicable.

Referring to FIG. 2, the wireless protocol structure of the LTE systemmay include packet data convergence protocols (PDCPs) 205 and 240, radiolink controls (RLCs) 210 and 235, medium access controls (MACs) 215 and230, and physical layers (PHYs) 220 and 225 respectively in a UE and anLTE eNB. The PDCPs 205 and 240 may perform operations such as IP headercompression/decompression. The main functions of the PDCPs 205 and 240may be summarized as follows.

-   -   Header compression and decompression (robust header compression        (ROHC) only))    -   Transfer of user data    -   In-sequence delivery of upper layer PDUs at a PDCP        re-establishment procedure for RLC acknowledged mode (AM)    -   For split bearers in DC (RLC AM): PDCP PDU routing for        transmission and PDCP PDU reordering for reception    -   Duplicate detection of lower layer SDUs at a PDCP        re-establishment procedure for RLC AM    -   Retransmission function PDCP SDUs at handover and for split        bearers in DC, PDCP PDUs at PDCP data-recovery procedure, for        RLC AM    -   Ciphering and deciphering functions    -   Timer-based SDU discard in an uplink

The RLCs 210 and 235 may reconfigure a PDCP PDU to an appropriate sizeto perform an automatic repeat request (ARQ) operation or the like. Themain functions of the RLCs 210 and 235 may be summarized as follows.

-   -   Transfer of upper layer PDUs    -   Error correction through an ARQ (only for AM data transfer)    -   Concatenation, segmentation and reassembly of RLC SDUs (only for        unacknowledged mode (UM) and AM data transfer)    -   Re-segmentation of RLC data PDUs (only for AM data transfer)    -   Reordering of RLC data PDUs (only for UM and AM data transfer)    -   Duplicate detection (only for UM and AM data transfer)    -   Protocol error detection (only for AM data transfer)    -   RLC SDU discard (only for UM and AM data transfer)    -   RLC re-establishment

The MACs 215 and 230 may be connected to a plurality of RLC layerdevices configured in one UE, and may perform operations of multiplexingRLC PDUs into MAC PDUs and demultiplexing RLC PDUs from MAC PDUs.

The main functions of the MACs 215 and 230 may be summarized as follows.

-   -   Mapping between logical channels and transport channels    -   Multiplexing/demultiplexing of MAC SDUs belonging to one or        different logical channels into/from transport blocks (TB)        delivered to/from the physical layer on transport channels    -   Scheduling information reporting    -   Error correction through HARQ    -   Priority handling between logical channels of one UE    -   Priority handling between UEs by means of dynamic scheduling    -   MBMS service identification    -   Transport format selection    -   Padding

The PHYs 220 and 225 may perform channel encoding and modulation onupper layer data and transmit OFDM symbols via a wireless channel byconverting the upper layer data to the OFDM symbols, or may demodulateOFDM symbols received via the wireless channel, perform channel decodingon the OFDM symbols and transmit decoded data to an upper layer.

FIG. 3 illustrates a structure of a next generation mobile communicationsystem to which embodiments of the present disclosure are applicable.

Referring to FIG. 3, a RAN of the next generation mobile communicationsystem (hereinafter, referred to as new radio (NR) or 5G) may include anew radio node B (NR gNB or NR base station) 310 and a new radio corenetwork (NR CN) 305. A new radio UE (NR UE or equipment) 315 may accessan external network through the NR gNB 310 and the NR CN 305.

In FIG. 3, the NR gNB 310 may correspond to an eNB of an existing LTEsystem. The NR gNB 310 may be connected to the NR UE 315 via a wirelesschannel and may provide a better service than the existing Node B. Inthe next generation mobile communication system, because all usertraffic is serviced through a shared channel, there is a need for adevice for collecting and scheduling status information such as a bufferstatus of UEs, an available transmission power status, and a channelstatus, which may be performed by the NR NB 310. One NR gNB may controlmultiple cells.

To implement ultra high-speed data transmission compared with thecurrent LTE, an existing maximum bandwidth may be given, and abeam-forming technique may be additionally applied by using OFDM as aradio access technique. Also, AMC may be applied, as described above.

The NR CN 305 performs functions such as a mobility support function, abearer setup function, and a QoS setup function, and performs variouscontrol functions as well as a mobility management function with respectto the NR UE 315, and may be connected to a plurality of base stations.The next generation mobile communication system may interact with theexisting LTE system. The NR CN 305 may be connected to the MME 325through a network interface. The MME 325 may be connected to a gNB 330which is an existing base station.

FIG. 4 illustrates a wireless protocol structure of a next generationmobile communication system to which embodiments of the presentdisclosure are applicable.

Referring to FIG. 4, the wireless protocol structure of the nextgeneration mobile communication system may include NR PDCPs 405 and 440,NR RLCs 410 and 435, NR MACs 415 and 430, and NR PHYs 420 and 425 in aUE and an NR gNB, respectively.

The main functions of the NR PDCPs 405 and 440 may include thefollowing.

-   -   Header compression and decompression (ROHC only)    -   Transfer of user data    -   In-sequence delivery of upper layer PDUs    -   PDCP PDU reordering for reception    -   Duplicate detection of lower layer SDUs    -   Retransmission of PDCP SDUs    -   Ciphering and deciphering function    -   Timer-based SDU discard in uplink

In the foregoing, reordering for reception of the NR PDCPs 405 and 440may represent sequential reordering of PDCP PDUs received from a lowerlayer based on PDCP sequence number (SN), and may include at least oneof delivering data to an upper layer in the reordered order, reorderingand recording lost PDCP PDUs, transmitting a status report regarding thelost PDCP PDUs to a transmitter, and requesting retransmission of thelost PDCP PDUs.

The main functions of the NR RLCs 410 and 435 may include the following.

-   -   Transfer of an upper layer PDU    -   In-sequence delivery of upper layer PDUs    -   Out-of-sequence delivery of upper layer PDUs    -   Error correction through ARQ    -   Concatenation, segmentation and reassembly of RLC SDUs    -   Re-segmentation of RLC data PDUs    -   Reordering of RLC data PDUs    -   Duplicate detection function    -   Protocol error detection    -   RLC SDU discard    -   RLC re-establishment

The in-sequence delivery of the NR RLCs 410 and 435 may representsequentially delivering RLC SDUs received from a lower layer to an upperlayer, and when one RLC SDU which has been segmented into a plurality ofRLC SDUs is received, may include reassembling and delivering theplurality of RLC SDUs. The sequential delivery may include at least oneof reordering the received RLC PDUs based on RLC sequence number (SN) orPDCP SN, reordering and recording lost RLC PDUs, transmitting a statusreport regarding the lost RLC PDUs to a transmitter, and requestingretransmission of the lost RLC PDUs. When there is a lost RLC SDU, thesequential delivery may include sequentially delivering only the RLCSDUs before the lost RLC SDU to the upper layer, and sequentiallydelivering, to the upper layer, all RLC SDUs received before apredetermined timer starts even when there is the lost RLC SDU when thetimer expired, or may include sequentially delivering all RLC SDUsreceived up to now to the upper layer even when there is the lost RLCSDU and the timer expired.

The NR RLCs 410 and 435 may process the RLC PDUs in the order ofreception (in the order of arrival irrespective of the order of sequencenumbers), and deliver the RLC PDUs to the NR PDCPs 405 and 440out-of-sequence delivery. In the case of a segment, the NR RLCs 410 and435 may receive segments stored in a buffer or to be received at a latertime, reconfigure the segments into one RLC PDU, and then process anddeliver the RLC PDU to the NR PDCPs 405 and 440. The NR RLCs 410 and 435may not include a concatenation function which may be performed by theNR MACs 415 and 430 or may be replaced by a multiplexing function of theNR MACs 415 and 430.

The out-of-sequence delivery of the NR RLCs 410 and 435 may representdelivering the RLC SDUs received from the lower layer directly to theupper layer, regardless of the order. When one RLC SDU which has beensegmented into a plurality of RLC SDUs is received, the out-of-sequencedelivery may include reassembling and delivering the plurality of RLCSDUs, storing an RLC SN or PDCP SN of the received RLC PDUs and orderingthereof, and recording of lost RLC PDUs.

The NR MACs 415 and 430 may be connected to a plurality of NR RLCsconfigured in one UE, and the main functions of the NR MACs 415 and 430may include the following.

-   -   Mapping between logical channels and transport channels.    -   Multiplexing/demultiplexing of MAC SDUs    -   Scheduling information reporting function    -   Error correction through HARQ    -   Priority handling between logical channels of one UE    -   Priority handling between UEs by means of dynamic scheduling    -   MBMS service identification    -   Transport format selection    -   Padding

The NR PHYs 420 and 425 may perform channel encoding and modulation onupper layer data and transmit OFDM symbols via a wireless channel byconverting the upper layer data into the OFDM symbols. The NR PHYs 420and 425 may demodulate OFDM symbols received via the wireless channel,perform channel decoding on the OFDM symbols and transmit decoded datato the upper layer.

FIG. 5 illustrates a structure for processing data in an LTE system,according to an embodiment.

As shown in FIG. 5, in the LTE system, a PDCP layer apparatus and an RLClayer apparatus may perform data processing for each logical channel.That is, a logical channel 1 LCID1 505 and a logical channel 2 LCID2 510may have different PDCP layer apparatuses and RLC layer apparatuses andperform independent data processing. Then, an RLC PDU generated from anRLC layer apparatus of each logical channel may be delivered to a MAClayer apparatus and may be configured as one MAC PDU, and then may betransmitted to a receiving end. In the LTE system, the PDCP layerapparatus, the RLC layer apparatus, and the MAC layer apparatus mayinclude the functions described above with reference to FIG. 4, and mayperform operations corresponding thereto.

The LTE system is characterized in that the RLC layer apparatusconcatenates PDCP PDUs. The LTE system is characterized in its structurein which all MAC subheaders are located in the front and a MAC SDU partis located in the rear of the MAC PDU as in a MAC PDU structure 525shown in FIG. 5. Due to the above-described characteristics, in the LTEsystem, the RLC layer apparatus may not perform or prepare dataprocessing before an uplink grant 530 is received.

As shown in FIG. 5, when the uplink grant 530 is received, a UE maygenerate the RLC PDUs by concatenating PDCP PDUs received from the PDCPlayer apparatus in accordance with the uplink grant 530. After the UEreceives the uplink grant 530 from a base station via the MAC layerapparatus, the UE may perform logical channel prioritization (LCP) andallocate the uplink grant 530 to each logical channel. That is, theuplink grant 530 may be allocated from the MAC layer apparatus. When thesize of the PDCP PDUs to be concatenated does not match the uplink grant530, the RLC layer apparatus may perform, for each logical channel, asegmentation procedure to match the PDCP PDUs with the uplink grant 530.Each RLC layer apparatus may configure an RLC header by using theconcatenated PDCP PDUs and send a completed RLC PDU to the MAC layerapparatus, which may configure the RLC PDUs (MAC SDUs) received from therespective RLC layer apparatuses as one MAC PDU to send one MAC PDU to aPHY layer apparatus for transmission. When the RLC layer apparatusperforms a segmentation operation when configuring the RLC header, andsegmented information is included in the RLC header, length informationof each of the concatenated PDCP PDUs may be included in the RLC headerfor reassembly at the receiving end.

As described above, in the LTE system, the data processing of the RLClayer apparatus, the MAC layer apparatus, and the PHY layer apparatusmay be started when the uplink grant 530 is received.

FIG. 6 illustrates a structure for processing data in a next generationmobile communication system according to an embodiment.

As shown in FIG. 6, in the next generation mobile communication system,data processing of a PDCP layer and an RLC layer may be performed foreach logical channel. That is, a logical channel D1 605 and a logicalchannel D2 610 may have different PDCP layer apparatuses and RLC layerapparatuses and perform independent data processing. An RLC PDUgenerated from an RLC layer apparatus 615 of each logical channel may bedelivered to a MAC layer apparatus 620 and may be configured as one MACPDU, and then may be transmitted to a receiving end. In the nextgeneration mobile communication system, a PDCP layer apparatus, the RLClayer apparatus 615, and the MAC layer apparatus 620 may include thefunctions described above with reference to FIG. 4 and performoperations corresponding thereto.

The next generation mobile communication system is characterized in thatthe RLC layer apparatus 615 does not concatenate PDCP PDUs. As shown inFIG. 6, the next generation mobile communication system is characterizedin its structure in which a MAC PDU structure 625 has a MAC subheaderfor each MAC SDU, that is, a structure in which a MAC subheader and aMAC SDU unit are repeated.

Therefore, in the next generation mobile communication system,pre-processing may be performed on data before an uplink grant 630 isreceived. That is, when a UE receives an IP packet from the PDCP layerapparatus before the UE receives the uplink grant 630, the UE mayperform PDCP processing such as ciphering and integrity protection onthe IP packet, generate a PDCP header and a PDCP PDU, and deliver thegenerated PDCP PDU to the RLC layer apparatus 615 to configure an RLCheader and an RLC PDU, and deliver the RLC PDU to the MAC layerapparatus 620 to previously configure the MAC subheader and the MAC SDU.

However, this is only an example, and the UE may alternatively performdata pre-processing only to the RLC layer apparatus 615 and process thedata in the MAC layer apparatus 620 when the uplink grant 630 isreceived. Alternatively, the UE may perform data pre-processing only onone of the PDCP header, the RLC header, and the MAC header to generateand separately process the headers. That is, before the UE receives theuplink grant 630, the UE may separately perform data pre-processing onheaders to generate the headers, and when the UE receives the uplinkgrant 630, the UE may concatenate the headers and the data to configurethe PDCP PDU, the RLC PDU, or the MAC PDU.

When data pre-processing is not implemented in the UE, data processingmay be performed after receiving the uplink grant 630 as in the LTEsystem. That is, when the uplink grant 630 is allocated to each logicalchannel after an LCP procedure is performed, the UE may configure thePDCP header by considering the size of the allocated LCP, generate thePDCP PDU, configure the RLC header to generate the RLC PDU, andconfigure the MAC subheader and the MAC SDU. When data pre-processing isnot implemented in the next generation mobile communication system, adifference between the next generation mobile communication system andthe LTE system is that the RLC layer apparatus 615 does not concatenatedata.

In the next generation mobile communication system, when the UE receivesthe uplink grant 630, the UE may configure the MAC PDU by fetching MACsubheaders and MAC SDUs corresponding to the size of the uplink grant630. Alternatively, when the UE performs data pre-processing in the RLClayer apparatus 615, the MAC layer apparatus 620 may receive the RLC PDUfrom each RLC layer apparatus 615 and configure and multiplex the MACsubheader and the MAC SDUs in accordance with the uplink grant 630 toconfigure the MAC PDU. The MAC subheader may also be pre-processed.

When the uplink grant 630 is insufficient, the UE may perform asegmentation operation for efficient use of all the uplink grant 630.When the segmentation operation is performed, the RLC header and the MACheader corresponding to the segmentation operation may be updated (640).For example, the segmentation information or length information may beincluded in the RLC header, and an L field corresponding to the lengthinformation of the MAC header may be updated.

Therefore, when reception of the uplink grant 630 in the next generationmobile communication system and reception of an uplink grant 645 in theLTE system are simultaneously performed, the next generation mobilecommunication system may realize a reduced processing time 635.

The RLC layer apparatus 615 and the PDCP layer apparatus may use onecommon sequence number when necessary or when set in a network.

The pre-processing operation may be performed for each logical channel,and, according to an embodiment, the RLC PDUs pre-processed for eachlogical channel may be further pre-processed to MAC SDUs and MACsubheaders in the MAC layer apparatus 620. The MAC layer apparatus 620may also perform data processing only when the uplink grant 630 isallocated, or the MAC subheader may be previously generated.

When the MAC layer apparatus 620 receives the uplink grant 630, the UEmay allocate the uplink grant 630 to each logical channel and multiplexthe pre-generated MAC SDUs and MAC subheaders.

When the MAC layer apparatus 620 receives the uplink grant 630 from abase station, the LCP operation may be performed, and accordingly, theMAC layer apparatus 620 may be allocated to each logical channel. TheMAC layer apparatus 620 may perform data pre-processing for each logicalchannel, configure the pre-generated MAC SDUs and MAC subheaders inaccordance with the uplink grant 630 for each logical channel, andmultiplex data for each logical channel to configure one MAC PDU anddeliver the MAC PDU to a PHY layer apparatus.

When the uplink grant 630 allocated to each logical channel isinsufficient, the MAC layer apparatus 620 may request the RLC layerapparatus 615 to segment data. When a segmentation operation isperformed by the RLC layer apparatus 615, information about segmentationis included in the header such that the header is updated and deliveredto the MAC layer apparatus 620. The MAC layer apparatus 620 may update aMAC header corresponding to and based on the delivered header.

As described above, the next generation mobile communication system ischaracterized in that data processing of the PDCP layer apparatus, theRLC layer apparatus 615, or the MAC layer apparatus 620 may be performedbefore the uplink grant 630 is received. When data pre-processing is notimplemented, the next generation mobile communication system may performdata processing after the uplink grant 630 is received, as in the LTEsystem. That is, when the uplink grant 630 is allocated to each logicalchannel after the LCP procedure is performed, the next generation mobilecommunication system may configure the PDCP header by considering thesize of the allocated uplink grant 630 to generate the PDCP PDU,configure the RLC header to generate the RLC PDU, and configure the MACsubheader and the MAC SDU. When data pre-processing is not implementedin the next generation mobile communication system, the RLC layerapparatus 615 does not concatenate data, in contrast with the LTEsystem.

The procedure for performing data pre-processing in the presentdisclosure may be applied to the following first, second, and thirdembodiments of the present disclosure.

First Embodiment of Data Pre-processing: Each PDCP layer apparatus maycipher a PDCP SDU (an IP packet or a data packet), perform integrityprotection when necessary, and generate a PDCP header, and each RLClayer apparatus may allocate an RLC sequence number, set a segmentationinformation (SI) field, and configure an RLC header to complete datapre-processing. When a MAC layer apparatus is satisfied with respect toa predetermined condition and instructs each RLC layer apparatus, inorder for each MAC layer apparatus to process a data pre-processed RLCPDU, each RLC layer apparatus may set a length L field corresponding tothe size of the RLC PDU, set a logical channel identifier (LCID) foreach RLC layer apparatus, configure a MAC header, configure andmultiplex each MAC subheader and MAC SDU to configure the MAC PDU inaccordance with the size of an uplink grant. The predetermined conditionof the MAC layer apparatus may be reception of the uplink grant from thebase station, and when the MAC layer apparatus receives the uplinkgrant, the MAC layer apparatus may instruct each RLC layer apparatus todeliver the data pre-processed RLC PDUs to the MAC layer apparatus.

Second Embodiment of Data Pre-Processing: Each PDCP header and RLCheader may be separately generated, stored and managed when the firstembodiment of data pre-processing is performed. When it is necessary toperform a segmentation operation due to a shortage of grant after theuplink grant is received, the UE may update an SI field of the generatedRLC header (01 in a first segment, 10 in a last segment, and 11 not ineither the first segment or the last segment), when necessary, a segmentoffset (SO) field may be dynamically added to the RLC header. Forexample, when it is not the first segment, the UE may add a 2-byte sizeSO field and instruct an offset.

Third Embodiment of Data Pre-processing: The first embodiment of datapre-processing may be performed, but a UE may perform data processing ofa MAC layer apparatus before an uplink grant is received. At this time,the UE may separately generate, store, and manage each PDCP header, eachRLC header, and each MAC header. When it is necessary to perform asegmentation operation due to a shortage of an uplink grant after theuplink grant is received, the UE may update an SI field of the generatedRLC header (01 in a first segment, 10 in a last segment, and 11 not inboth the first segment and the last segment), when necessary, a segmentoffset (SO) field may be dynamically added to the RLC header. Forexample, the UE may add a 2-byte size SO field and instruct an offsetwhen it is not the first segment.

FIG. 7 illustrates a system to which a data transmission method of adual access UE is applicable according to an embodiment.

As shown in FIG. 7, in dual access technology, a UE increases a datatransmission rate in the downlink and uplink through a connectionbetween a master cell group (MCG) base station and a secondary cellgroup (SCG) base station. The MCG base station may transmit and receivemost of control signals and determine connection, change, and release ofthe SCG base station.

As shown in FIG. 7, the MCG base station may be an NR base station or anLTE base station. The SCG base station may also be the NR base stationor the LTE base station. A data transmission method according to anembodiment may be applied to the following 4 dual access technologyenvironments.

1. An LTE-LTE DC dual access environment 705 in which the LTE basestation is the MCG base station and the LTE base station is the SCG basestation:

2. An LTE-NR DC dual access environment 710 in which the LTE basestation is the MCG base station and the NR base station is the SCG basestation;

3. An NR-LTE DC dual access environment 715 in which the NR base stationis the MCG base station and the LTE base station is the SCG basestation;

4. An NR-NR DC dual access environment 720 in which the NR base stationis the MCG base station and the NR base station is the SCG base station.

FIG. 8 illustrates a procedure in which a UE to which a dual accesstechnology is applied receives an SCG change message or an SCGconnection-release message according to an embodiment.

In FIG. 8, when an SCG is changed (805), and the UE receives the SCGchange message from an MCG base station, the UE may change a secondarycell indicated by the SCG change message to a new secondary cell. Whenthe SCG is released (810), and the UE receives the SCGconnection-release message from the MCG base station, the UE may releaseconnection to a secondary cell indicated in the SCG connection-releasemessage. The SCG change message and the SCG connection-release messageare radio resource control (RRC) connection reconfiguration messages(RRC messages) and may be transmitted from the MCG base station to theUE.

When the UE receives the SCG change message or the SCGconnection-release message, the UE may re-establish an RLC layerapparatus or a MAC layer apparatus, which corresponds to an SCGindicated in the RRC message, and perform a PDCP data recoveryprocedure.

A first embodiment of the PDCP data recovery procedure with respect touplink data transmission of the UE when the UE receives the SCG changemessage or the SCG connection-release message according to an embodimentis as follows.

When the UE receives the SCG change message or the SCGconnection-release message and thus receives, from an upper layer, acommand to perform the PDCP data recovery procedure, the UE mayre-establish the RLC layer apparatus or the MAC layer apparatus whichcorresponds to the SCG indicated in the RRC message, and may perform thePDCP data recovery procedure, as follows.

1. When a PDCP status report is received from a base station, the UE maydiscard data (PDCP PDU or PDCP SDU) of which successful delivery isacknowledged in the PDCP status report, and perform retransmission ofdata of which successful delivery is not acknowledged. Theretransmission may be performed through a link that is currently set andis capable of transmitting and receiving data. For example, theretransmission may be selectively performed via any one of an MCG link815 and a newly changed SCG link 825. In this regard, a link to be usedfor the retransmission may be determined according to the PDCPimplementation.

2. When the PDCP status report is not received from the base station,the UE may retransmit, to the re-established RLC layer apparatus, thePDCP PDUs that have been transmitted. When the UE retransmits the PDCPPDUs, the UE may retransmit all PDCP PDUs sequentially starting from afirst PDCP PDU of which successful delivery is not acknowledged from alower layer. The retransmission may be performed through a link that iscurrently set and is capable of transmitting and receiving data, such asany one of the MCG link 815 and the newly changed SCG link 825. In thisregard, a link to be used for the retransmission may be determinedaccording to the PDCP implementation.

In the example of the first embodiment, it is assumed that the UEtransmits PDCP PDUs corresponding to PDCP sequence numbers 0, 1, 2, and3 to the uplink in the PDCP layer apparatus through the MCG link 815 andPDCP PDUs corresponding to PDCP sequence numbers 4, 5, 6, 7, 8, and 9through the SCG link 820. It is assumed that the UE receives an RLC ACKcorresponding to the PDCP sequence numbers 1 and 2 from the MCG link 815and an RLC ACK corresponding to the PDCP sequence numbers 4, 6, 7, 8,and 9 from the SCG link 820.

When the UE receives the SCG change message or the SCGconnection-release message from the base station, the UE mayre-establish the RLC layer apparatus or the MAC layer apparatus whichcorresponds to the SCG indicated in the RRC message, and may perform thePDCP data recovery procedure, during which the UE may check whether thePDCP status report is included in the RRC message. When the PDCP statusreport is included in the RRC message, the UE may discard the data (PDCPPDU or PDCP SDU) of which successful delivery is acknowledged in thePDCP status report and perform retransmission of the data of whichsuccessful delivery is not acknowledged. Retransmission may be performedthrough a link that is currently set and is capable of transmitting andreceiving data. For example, retransmission may be selectively performedvia any one of the MCG link 815 and the newly changed SCG link 825. Inthis regard, the link to be used for retransmission may be determinedaccording to the PDCP implementation.

When the PDCP status report is not included in the RRC message, the UEmay re-establish the RLC layer apparatus or the MAC layer apparatuswhich corresponds to the SCG indicated in the RRC message, and performthe PDCP data recovery procedure. The UE may perform retransmission ofthe PDCP PDUs corresponding to the PDCP sequence numbers 4, 5, 6, 7, 8,and 9 transmitted to the re-established RLC layer apparatus. The UE mayperform retransmission of all PDCP PDUs sequentially from the first PDCPPDU of which successful delivery is not acknowledged from the lowerlayer. That is, since the PDCP PDU corresponding to the PDCP sequencenumber 5 from the re-established RLC layer apparatus is the first PDCPPDU of which successful delivery (RLC ACK) is not acknowledged, the UEmay perform retransmission of the PDCP PDUs corresponding to the PDCPsequence numbers 4, 5, 6, 7, 8, and 9 through a link that is currentlyset and is capable of transmitting and receiving data. For example,retransmission may be selectively performed via any one of the MCG link815 and the newly changed SCG link 825. In this regard, the link to beused for retransmission may be determined according to the PDCPimplementation.

Table 1 below illustrates PDCP PDUs delivered to each base station inthe example of the first embodiment.

TABLE 1 PDCP SN 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 RLC1 0 0 RLC2 0 00 0 0

A second embodiment of the PDCP data recovery procedure of the UE forreducing a data transmission delay with respect to the uplink datatransmission when the UE receives the SCG change message or the SCGconnection-release message according to an embodiment is as follows.

When the UE receives the SCG change message or the SCGconnection-release message and thus, the UE receives, from an upperlayer, a command to perform the PDCP data recovery procedure, the UE mayre-establish the RLC layer apparatus or the MAC layer apparatus whichcorresponds to the SCG indicated in the RRC message, and may perform thefollowing PDCP data recovery procedure.

1. When a PDCP status report is received from a base station, the UE maydiscard data (PDCP PDU or PDCP SDU) of which successful delivery isacknowledged in the PDCP status report and perform retransmission ofdata of which successful delivery is not acknowledged, through a linkthat is currently set and is capable of transmitting and receiving data.For example, the retransmission may be selectively performed via any oneof the MCG link 815 and the newly changed SCG link 825. In this regard,a link to be used for the retransmission may be determined according tothe PDCP implementation.

2. When the PDCP status report is not received from the base station,the UE may retransmit the PDCP PDUs that have been transmitted to there-established RLC layer apparatus, by retransmitting all PDCP PDUssequentially starting from a first PDCP PDU of which successful deliveryis not acknowledged from a lower layer. The retransmission may beperformed through a link that is currently set and is capable oftransmitting and receiving data, such as any one of the MCG link 815 andthe newly changed SCG link 825. In this regard, a link to be used forthe retransmission may be determined according to the PDCPimplementation.

In the example of the second embodiment, it is assumed that the UEtransmits PDCP PDUs corresponding to PDCP sequence numbers 0, 1, 2, and3 to the uplink in the PDCP layer apparatus through the MCG link 815 andPDCP PDUs corresponding to PDCP sequence numbers 4, 5, 6, 7, 8, and 9through the SCG link 820, and that the UE receives an RLC ACKcorresponding to the PDCP sequence numbers 1 and 2 from the MCG link 815and an RLC ACK corresponding to the PDCP sequence numbers 4, 6, 7, 8,and 9 from the SCG link 820.

When the UE receives the SCG change message or the SCGconnection-release message from the base station, the UE mayre-establish the RLC layer apparatus or the MAC layer apparatus whichcorresponds to the SCG indicated in the RRC message, perform the PDCPdata recovery procedure, and check whether the PDCP status report isincluded in the RRC message.

The UE may discard the data (PDCP PDU or PDCP SDU) of which successfuldelivery is acknowledged in the PDCP status report and performretransmission of the data of which successful delivery is notacknowledged, through a link that is currently set and is capable oftransmitting and receiving data. For example, retransmission may beselectively performed via any one of the MCG link 815 and the newlychanged SCG link 825. In this regard, the link to be used forretransmission may be determined according to the PDCP implementation.When the PDCP status report is not included in the RRC message, the UEmay re-establish the RLC layer apparatus or the MAC layer apparatuswhich corresponds to the SCG indicated in the RRC message, and performthe PDCP data recovery procedure. The UE may perform retransmission ofthe PDCP PDUs corresponding to the PDCP sequence numbers 4, 5, 6, 7, 8,and 9 transmitted to the re-established RLC layer apparatus. When the UEperforms retransmission, the UE may perform retransmission of all PDCPPDUs sequentially from the first PDCP PDU of which successful deliveryis not acknowledged from the lower layer. That is, because the PDCP PDUcorresponding to the PDCP sequence number 5 from the re-established RLClayer apparatus is the first PDCP PDU of which successful delivery (RLCACK) is not acknowledged, the UE may perform retransmission of the PDCPPDU corresponding to the PDCP sequence number 5. At this time,retransmission may be performed through a link that is currently set andis capable of transmitting and receiving data, such as any one of theMCG link 815 and the newly changed SCG link 825. In this regard, thelink to be used for retransmission may be determined according to thePDCP implementation.

In the above example, loss may not occur even when selectiveretransmission is performed. In view of a reception end, a receiving RLClayer apparatus has successfully received the PDCP PDUs corresponding tothe PDCP sequence numbers 4, 6, 7, 8, and 9, and therefore instructs thesuccessful delivery (RLC ACK) when sending the RLC status report to atransmission end.

When the receiving RLC layer apparatus is an LTE RLC layer apparatus,because the LTE RLC layer apparatus sequentially delivers the PDCP PDUsto the upper layer, the LTE RLC layer apparatus may not deliver datacorresponding to the PDCP sequence numbers 6, 7, 8, and 9 to a PDCPlayer. However, when the SCG is changed or connection-released, thereceiving RLC layer apparatus may be re-established, and in this regard,the RLC layer apparatus may remove the RLC header from the datacorresponding to the PDCP sequence numbers 6, 7, 8 and 9 which are notsequential, process the data, and then deliver the data to the PDCPlayer. Therefore, the receiving RLC layer apparatus has the PDCP PDUsfor which the successful delivery has been instructed by using RLC ACK,thereby reducing unnecessary retransmission. That is, the UE does notneed to retransmit all the PDCP PDUs from the first PDCP PDUs of whichsuccessful delivery is not acknowledged, and may perform selectiveretransmission of only PDCP PDUs of which successful delivery is notacknowledged with respect to RLC ACK.

When the receiving RLC layer apparatus is an NR RLC layer apparatus,since the NR RLC layer apparatus does not have an order deliveryfunction of sequentially delivering data to the upper layer and performsan out-of-order delivery function, data for which successful deliveryhas been instructed by using RLC ACK in the RLC status report may beimmediately delivered to the PDCP layer. Therefore, the RLC layerapparatus may remove the RLC header from the data corresponding to thePDCP sequence numbers 6, 7, 8 and 9 and process the data, and thendeliver the data to the PDCP layer. Therefore, the receiving RLC layerapparatus has the PDCP PDUs for which the successful delivery has beeninstructed by using RLC ACK, and the UE does not perform unnecessaryretransmission.

Consequently, when selective retransmission is performed based on thesuccessful delivery (RLC ACK) of the RLC layer apparatus disclosedabove, unnecessary retransmission and corresponding data transmissiondelay may be reduced.

A third embodiment in which a data transmission delay is reduced when aUE receives the SCG change message or the SCG connection-release messageaccording to the embodiment is as follows.

When the UE receives the SCG change message or the SCGconnection-release message, and thus, the UE receives, from an upperlayer, a command to perform the PDCP data recovery procedure, the UE mayre-establish the RLC layer apparatus or the MAC layer apparatus whichcorresponds to the SCG indicated in the RRC message, and perform thePDCP data recovery procedure. In the following third embodiment, whenthe base station sends the SCG change message or the SCGconnection-release message, the base station may always include the PDCPstatus report to instruct PDCP sequence numbers of data that a PDCPlayer apparatus successfully received and did not successfully receive.Accordingly, when the UE receives the SCG change message or the SCGconnection-release message, the UE may always receive the PDCP statusreport and perform selective retransmission, such that unnecessaryretransmission and corresponding transmission delay of new data may bereduced.

FIG. 9 illustrates a procedure in which an UE detects and declares aradio link failure (RLF) in an SCG according to an embodiment.

In FIG. 9, the dual-accessed UE may declare the RLF when the intensityof signal is constantly low in an SCG link 915 during datatransmission/reception between an MCG link 910 and the SCG link 915. TheUE may declare the RLF when the maximum number of retransmissions isexceeded in an RLC layer apparatus. However, this is merely an examplein which the UE declares the RLF, and the present disclosure is notlimited to the above-described example.

When the UE determines the RLF with respect to the SCG (905), the UEneeds to report a base station at which the RLF occurs with respect tothe SCG. The UE may report the SCG RLF through the MCG link 910 and maystop data transmission with respect to the SCG and stand by until thebase station instructs an SCG change message or an SCGconnection-release message.

Therefore, in the above procedure, while the UE waits for an instruction(SCG change or connection-release) from the base station, retransmissionof data lost in the SCG link 915 may not be performed. Therefore, areceiving PDCP layer apparatus may wait for the lost data for apredetermined time period (for example, for a value of an orderreordering timer), which may cause the data transmission delay.

The first, second, or third embodiment for reducing the transmissiondelay caused when the UE receives the SCG change message or the SCGconnection-release message may be more applicable to, in particular, asplit bearer set to the UE configured to support a dual accessenvironment, and may also be applied to when the UE receives a logicalchannel release message or a logical channel change message or a logicalchannel add message.

In the embodiments described above, the SCG change message and the SCGrelease message may instruct an operation of changing or releasing allbearers and logical channels set in the SCG. However, the logicalchannel release message or the logical channel change message or thelogical channel add message may release or change or add at least one ofsome logical channels, RLC layer apparatuses and MAC layer apparatusesof an MCG or the SCG. For example, when three split bearers are set inthe UE, that is, split bearer 1, split bearer 2, and split bearer 3 areset over the MCG and the SCG, a base station may transmit the logicalchannel release message to the UE and release at least one of a logicalchannel, an RLC layer apparatus, and a MAC layer apparatus correspondingto an SCG of the split bearer 2, and change the released one to a datawireless bearer (DRB). A logical channel may be changed or added to aspecific split bearer or DRB through the logical channel change messageor the logical channel add message. In the above example, when the SCGrelease message is transmitted to the UE, connections of all three splitbearers may be released. Accordingly, the base station may usefullyinstruct the UE with a more specific command through the logical channelrelease message, the logical channel change message, or the logicalchannel add message, so as to change a bearer type for each bearer ofthe UE. That is, the logical channel release message, the logicalchannel change message or the logical channel add message may be used tochange the DRB to the split bearer or the split bearer to the DRB.

When the UE receives the logical channel release message, the logicalchannel change message, or the logical channel add message, and themessage instructs to release or change or add at least one of a logicalchannel, an RLC layer apparatus and a MAC layer apparatus correspondingto the SCG of the split bearer (or the DRB), the first embodiment, thesecond embodiment or the third embodiment for reducing the transmissiondelay may be equally applied thereto.

The UE according to a fourth embodiment may perform the PDCP datarecovery procedure of reducing the data transmission delay when the UEdetects an RLF in an SCG link in uplink data transmission.

When the UE detects the RLF in the SCG link, the UE may report an SCGRLF with respect to the link to the base station via an MCG link, stopdata transmission corresponding to the SCG, and immediately perform anext PDCP data recovery procedure.

The UE may perform retransmission of PDCP PDUs that have beentransmitted to an RLC layer apparatus from which the RLF is detected.When the UE performs retransmission, the UE may perform selectiveretransmission on only data (PDCP PDU or PDCP SDU) of which successfuldelivery (RLC ACK) is not acknowledged in a lower layer, andretransmission may be performed through the MCG link 910 that iscurrently set and is capable of transmitting and receiving data.

When the UE receives the SCG change message or the SCG release messagefrom the base station, the UE may re-establish the RLC layer apparatusor the MAC layer apparatus which corresponds to the SCG instructed inthe RRC message. The PDCP data recovery procedure is not performed againsince the PDCP data recovery procedure has already been performed.

The above-described example is a method that may be applied to when theUE detects the RLF in the SCG link, and may reduce data transmissiondelay. After the UE reports the SCG RLF, the UE may receive one commandof the SCG change message and the SCG release message from the basestation, and is required to perform the PDCP data recovery procedure.Therefore, when the RLF is detected, the UE may perform the PDCP datarecovery procedure immediately, such that the data transmission delaymay be reduced.

The fourth embodiment described above corresponds to a procedureperformed when the UE detects the RLF in the SCG link, and the firstembodiment, the second embodiment, or the third embodiment describedabove may be applied to when the UE does not detect the RLF in the SCGlink but receives the SCG change message or the SCG release message,

An example of the fourth embodiment is as follows.

It is assumed that the UE transmits PDCP PDUs corresponding to PDCPsequence numbers 0, 1, 2, and 3 to the uplink in the PDCP layerapparatus through the MCG link 910 and PDCP PDUs corresponding to PDCPsequence numbers 4, 5, 6, 7, 8, and 9 through SCG link 915. Also, it isassumed that the UE receives an RLC ACK corresponding to the PDCPsequence numbers 1 and 2 from the MCG link 910 and RLC ACK correspondingto the PDCP sequence numbers 4, 6, 7, 8, and 9 from the SCG link 915.

When the UE receives the RLF with respect to the SCG link 915, the UEmay report the RLF with respect to the SCG link 915 via the MCG link910, wait for an instruction from the base station, and perform the PDCPdata recovery procedure. The UE may perform retransmission of the PDCPPDUs corresponding to the PDCP sequence numbers 4, 5, 6, 7, 8, and 9that have been transmitted to an RLC layer apparatus corresponding tothe SCG link 915 from which the RLF is detected, by performingretransmission of all PDCP PDUs sequentially from the first PDCP PDU ofwhich successful delivery (RLC ACK) is not acknowledged from the lowerlayer. That is, because the PDCP PDU corresponding to the PDCP sequencenumber 5 from the RLC layer apparatus corresponding to the SCG link 915from which the RLF is detected is the first PDCP PDU of which successfuldelivery (RLC ACK) is not acknowledged, the UE may performretransmission of the PDCP PDU corresponding to the PDCP sequence number5, through MCG link 910 that is currently set and is capable oftransmitting and receiving data.

In the above example, loss may not occur even when selectiveretransmission is performed. In view of a reception end, a receiving RLClayer apparatus has successfully received the PDCP PDUs corresponding tothe PDCP sequence numbers 4, 6, 7, 8, and 9, and therefore, instructsthe successful delivery (RLC ACK) when sending the RLC status report toa transmission end. When the receiving RLC layer apparatus is an LTE RLClayer apparatus, the LTE RLC layer apparatus delivers the PDCP PDUs thatare sequentially received to the upper layer, and does not deliver datacorresponding to the PDCP sequence numbers 6, 7, 8, and 9 to a PDCPlayer. However, when the SCG is changed or connection to the SCG isreleased, the receiving RLC layer apparatus may be re-established, andthe RLC layer apparatus may remove the RLC header from the datacorresponding to the PDCP sequence numbers 6, 7, 8 and 9 which are notsequential and process the data, and then deliver the data to the PDCPlayer. Because the receiving RLC layer apparatus has the PDCP PDUs forwhich the successful delivery has been instructed by using RLC ACK, theUE does not need to perform unnecessary retransmission. That is, the UEdoes not need to retransmit all the PDCP PDUs from the first PDCP PDUsof which successful delivery is not acknowledged, and may performselective retransmission of only PDCP PDUs of which successful deliveryis not acknowledged with respect to the RLC ACK.

When the receiving RLC layer apparatus is an NR RLC layer apparatus,since the NR RLC layer apparatus does not have an order deliveryfunction of sequentially delivering data to the upper layer and performsan out-of-order delivery function, data for which successful deliveryhas been instructed by using an RLC ACK in the RLC status report may beimmediately delivered to the PDCP layer. Therefore, the RLC layerapparatus may remove the RLC header from the data corresponding to thePDCP sequence numbers 6, 7, 8 and 9, process the data, and then deliverthe data to the PDCP layer. Therefore, the receiving RLC layer apparatushas the PDCP PDUs for which the successful delivery has been instructedby using RLC ACK, and the UE does not need to perform unnecessaryretransmission.

Consequently, unnecessary retransmission may be reduced when selectiveretransmission is performed based on the successful delivery (RLC ACK)of the RLC layer apparatus according to an embodiment, and the datatransmission delay may be reduced since the PDCP data recovery proceduremay be performed as soon as the RLF is detected.

A fifth embodiment of the PDCP data restoration procedure for reducingthe data transmission delay when the UE detects the RLF in the SCG linkduring the uplink data transmission is as follows. When the UE detectsthe RLF in the SCG link, the UE may report the SCG RLF with respect tothe link to the base station via the MCG link, and may stop datatransmission corresponding to the SCG and immediately perform a nextPDCP data recovery procedure.

The UE may perform retransmission of the PDCP PDUs that have beentransmitted to the RLC layer apparatus from which the RLF is detected,by performing retransmission of all PDCP PDUs sequentially from thefirst PDCP PDU of which successful delivery (RLC ACK) is notacknowledged from the lower layer. The retransmission may be performedthrough MCG link 910 that is currently set and is capable oftransmitting and receiving data.

When the UE receives the SCG change message or the SCG release messagefrom the base station, the UE may re-establish the RLC layer apparatusor the MAC layer apparatus which corresponds to the SCG instructed inthe RRC message. The PDCP data recovery procedure is not performed againsince the PDCP data recovery procedure has already been performed.

The fifth embodiment may be applied to when the UE detects the RLF inthe SCG link, and may reduce data transmission delay d. After the UEreports the SCG RLF, the UE may receive one command of the SCG changemessage and the SCG release message from the base station, and isrequired to perform the PDCP data recovery procedure. Therefore, whenthe RLF with respect to the SCG link is detected, the UE may immediatelyperform the PDCP data recovery procedure that is required to beperformed, such that the data transmission delay may be reduced.

The fifth embodiment described above corresponds to a procedureperformed when the UE detects the RLF in the SCG link, as does thefourth embodiment, and the first embodiment, the second embodiment, andthe third embodiment described above may be applied to when the UE doesnot detect the RLF in the SCG link but receives the SCG change messageor the SCG release message.

FIG. 10 illustrates a method of transmitting data when a UE receives anSCG change message or an SCG release message according to an embodiment.

In step 1010, the UE may receive an SCG change message or an SCGconnection-release message from a MCG base station. In the presentembodiment, it is assumed that the UE is dually accessed to the MCG basestation and an SCG base station and supports a bearer splitconfiguration.

In step 1020, the UE may determine whether a PDCP status report isreceived from the MCG base station. The UE may acknowledge data unitsreceived by the base station according to the PDCP status report, andthe data unit may be a PDCP PDU.

In step 1030, the UE may determine a data unit of which reception isacknowledged among at least one data unit transmitted from the UE, basedon whether the PDCP status report is received.

When the PDCP status report is received, the UE may determine the dataunit of which reception is acknowledged according to the PDCP statusreport. According to another example, when the PDCP status report is notreceived, the UE may determine a data unit of which reception isacknowledged from a lower layer.

In step 1040, the UE may selectively retransmit a data unit of whichreception is not acknowledged to the MCG base station, based on thedetermination. FIG. 11 illustrates a data transmission method of a UEaccording to a first embodiment.

In step 1110, the UE may receive an SCG change message or an SCGconnection-release message. When the UE receives the SCG change messageor the SCG connection-release message, the UE may re-establish an RLClayer apparatus or a MAC layer apparatus which corresponds to an SCGinstructed in an RRC message, and initiate a PDCP data recoveryprocedure.

In step 1120, the UE may determine whether a PDCP status report isreceived.

In step 1130, when the PDCP status report is received from the MCG basestation, the UE may discard data (PDCP PDU or PDCP SDU) of whichreception is acknowledged in the PDCP status report and performretransmission only on data of which reception is not acknowledged. Theretransmission may be performed through a link that is currently set andis capable of transmitting and receiving data, such as an MCG link or anewly changed SCG link, and in this regard, a link to be used for theretransmission may be determined according to the PDCP implementation.

In step 1140, when the PDCP status report is not received from the MCGbase station, the UE may perform retransmission of the PDCP PDUs thathave been transmitted, to the re-established RLC apparatus. When the UEperforms the retransmission, the UE may perform the retransmission ofall PDCP PDUs sequentially from a first PDCP PDU of which reception isnot acknowledged from a lower layer. The retransmission may be performedthrough a link that is currently set and is capable of transmitting andreceiving data, such as the MCG link or the newly changed SCG link. Inthis regard, a link to be used for the retransmission may be determinedaccording to the PDCP implementation.

FIG. 12 illustrates a data transmission method of a UE according to asecond embodiment.

In step 1210, the UE may receive an SCG change message or an SCGconnection-release message, may re-establish an RLC layer apparatus or aMAC layer apparatus which corresponds to an SCG instructed in an RRCmessage, and may perform a PDCP data recovery procedure.

In step 1220, the UE may determine whether a PDCP status report isreceived from a base station.

In step 1230, when the PDCP status report is received, the UE mayperform selective retransmission after the UE acknowledges received PDCPPDU through the PDCP status report. In more detail, the UE may discarddata (PDCP PDU or PDCP SDU) of which reception is acknowledged andperform retransmission of only data of which reception is notacknowledged through a link that is currently set and is capable oftransmitting and receiving data, such as an MCG link or a newly changedSCG link. In this regard, a link to be used for the retransmission maybe determined according to the PDCP implementation.

In step 1240, when the PDCP status report is not received, the UE mayperform selective retransmission after the UE acknowledges PDCP PDUreceived from a lower layer. The UE may perform retransmission of PDCPPDUs that have been transmitted, to the re-established RLC layerapparatus.

When the UE performs the retransmission, the UE may perform selectiveretransmission of only the data (PDCP PDU or PDCP SDU) of whichreception is not acknowledged in the lower layer, through a link that iscurrently set and is capable of transmitting and receiving data. Forexample, the UE may retransmit data via the MCG link or the newlychanged SCG link. In this regard, a link to be used for theretransmission may be determined according to the PDCP implementation.

FIG. 13 illustrates a method of selectively retransmitting data when aUE detects an RLF according to an embodiment.

In step 1310, the UE may detect the RLF in an SCG link, and may reportthe SCG RLF with respect to the SCG link to a base station via an MCGlink. The UE may stop transmission of data corresponding to an SCG.

In step 1320, the UE may initiate a PDCP data recovery procedure. The UEmay perform retransmission of PDCP PDUs that have been transmittedthrough an RLC layer apparatus from which the RLF is detected.

In step 1330, when the UE performs the retransmission according toinitiation of the recovery procedure, the UE may perform selectiveretransmission after the UE acknowledges PDCP PDUs received in a lowerlayer. In more detail, the UE may perform selective retransmission ofonly data (PDCP PDU or PDCP SDU) of which reception is not acknowledged,through a link that is currently set and is capable of transmitting andreceiving data, such as the MCG link.

When the UE receives an SCG change message or a SCG connection-releasemessage from the base station, the UE may re-establish an RLC layerapparatus or a MAC layer apparatus which corresponds to the SCGinstructed in an RRC message. The PDCP data recovery procedure is notperformed again since the PDCP data recovery procedure has already beenperformed.

FIG. 14 illustrates a method, performed by a UE, of sequentiallyretransmitting data of which reception is not acknowledged when an RLFis detected according to an embodiment.

In step 1410, the UE may detect the RLF in an SCG link, and may reportthe SCG RLF with respect to the SCG link to a base station via an MCGlink. The UE may stop transmission of data corresponding to a SCG.

In step 1420, the UE may initiate a PDCP data recovery procedure.

In step 1430, the UE may perform retransmission of all PDUs sequentiallyfrom a first PDU of which reception is not acknowledged from a lowerlayer. The UE may start the retransmission of the PDCP PDUs that havebeen transmitted to an RLC layer apparatus from which the RLF isdetected. The UE may perform the retransmission of all the PDCP PDUssequentially from the first PDCP PDUs of which reception is notacknowledged in the lower layer, through a link that is currently setand is capable of transmitting and receiving data, such as through theMCG link.

When the UE receives an SCG change message or an SCG connection-releasemessage from the base station, the UE may re-establish an RLC layerapparatus or a MAC layer apparatus which corresponds to the SCGinstructed in the RRC message. The PDCP data recovery procedure is notperformed again since the PDCP data recovery procedure has already beenperformed.

According to an embodiment, when the base station triggers the PDCP datarecovery procedure, the UE may perform selective retransmission on databearers that use an AM mode based on a successful deliveryacknowledgment (RLC ACK) of lower layer apparatuses (RLC layerapparatuses) when a PDCP layer apparatus performs the PDCP data recoveryprocedure. However, when the UE performs handover to another cell in onebase station, the UE may perform selective retransmission on signalingwireless bearers (SRBs) in a same manner as the above-described methodand proceed with the data recovery procedure to prevent data loss.

The base station may also define a new indicator to indicate whether toperform the PDCP data recovery procedure on the SRB. That is, the PDCPdata recovery procedure through the selective retransmission may beextended and applied to the SRB. When the UE performs handover toanother cell in one base station as described above, and when the PDCPdata recovery procedure is triggered, the UE may not performretransmission on bearers that use an UM mode but may perform datatransmission on data that is not yet transmitted from the PDCP layerapparatus (data that is not yet delivered to the lower layer) as if thedata is received from an upper layer.

The selective retransmission method according to the embodiment may beapplied to a PDCP re-establishment procedure. However, in the case ofhandover, the PDCP re-establishment procedure triggered by the basestation may cause data loss when selective retransmission is alwaysused, since it is not mandatory for a PDCP layer apparatus of a sourcebase station to deliver all successfully received data to a PDCP layerapparatus of a target base station. Therefore, even when the UEreceives, from the source base station, a report that the data issuccessfully received, it is necessary to retransmit, to the target basestation, data of which successful delivery is acknowledged by using anRLC ACK. That is, the UE may perform accumulated retransmission by whichdata corresponding to a first PDCP sequence number of which successfuldelivery is not acknowledged is sequentially retransmitted during thePDCP re-establishment procedure. Therefore, retransmission may beperformed even when there is data of which successful delivery isacknowledged (receiving RLC ACK) by the lower layer among data having asequence number greater than the first PDCP sequence number of whichsuccessful delivery is not acknowledged.

However, when the PDCP layer apparatus of the source base stationdelivers all the successfully received data to the PDCP layer apparatusof the target base station, the UE may perform the selectiveretransmission method according to the embodiment to prevent unnecessaryretransmission and waste of transmission resources. Thus, by definingthe new indicator, the base station may indicate whether to performselective retransmission or accumulated retransmission to the UE whenthe UE performs the PDCP re-establishment procedure. For example, theselective retransmission may be performed when there is the indicator,and the accumulated retransmission may be performed when there is noindicator. According to another example, when a 1-bit indicator isdefined as the indicator and indicates a value “True”, the selectiveretransmission may be performed, and when the 1-bit indicator indicatesa value “False”, the accumulated retransmission may be performed. Theindicator may also be defined in the RRC message, may be indicated inPDCP configuration information, and may be indicated via a handoverindication message or an RRC reset message.

FIG. 15 is a block diagram of a UE 1500 according to an embodiment.

Referring to FIG. 15, the UE 1500 may include a radio frequency (RF)processor 1510, a baseband processor 1520, a storage 1530, and acontroller 1540. However, components of FIG. 15 are only an example, andthe present disclosure is not limited thereto.

The RF processor 1510 may perform transmission and reception of a signalthrough a wireless channel by converting a band of the signal oramplifying the signal, for example. That is, the RF processor 1510 mayup-convert a baseband signal provided from the baseband processor 1520to an RF band signal, transmit the RF band signal through an antenna,and down-convert an RF band signal received through the antenna to abaseband signal. For example, the RF processor 1510 may include atransmission filter, a reception filter, an amplifier, a mixer, anoscillator, a digital-to-analog converter (DAC), and ananalog-to-digital converter (ADC). While only one antenna is shown inFIG. 15, the UE 1500 may have a plurality of antennas.

The RF processor 1510 may include a plurality of RF chains and mayperform beamforming by adjusting a phase and magnitude of each ofsignals transmitted and received via the plurality of antennas orantenna elements. The RF processor 1510 may perform a MIMO operation andmay receive a plurality of layers when performing the MIMO operation.The RF processor 1510 may perform reception beam sweeping byappropriately setting the plurality of antennas or the antenna elementsby the control of the controller 1540, or adjust a direction and widthof a received beam such that the received beam coordinates with atransmitted beam.

The baseband processor 1520 may perform conversion between the basebandsignal and a bit string according to a physical layer specification ofthe system. For example, upon transmitting data, the baseband processor1520 may generate complex symbols by encoding and modulating transmittedbit strings. Upon receiving data, the baseband processor 1520 maydemodulate and decode a baseband signal provided from the RF processor1510 to reconstruct received bit strings. For example, when data istransmitted according to an OFDM scheme, the baseband processor 1520 maygenerate complex symbols by encoding and modulating transmitted bitstrings, map the complex symbols to subcarriers, and configure OFDMsymbols by performing an inverse fast Fourier transform (IFFT) operationand inserting a cyclic prefix (CP).

Upon receiving data, the baseband processor 1520 may split the basebandsignal provided from the RF processor 1510 into OFDM symbol units andrestore the signals mapped to the subcarriers by performing a fastFourier transform (FFT) operation and then reconstruct the received bitstrings by performing demodulation and decoding.

The baseband processor 1520 and the RF processor 1510 may transmit andreceive signals as described above. Accordingly, the baseband processor1520 and the RF processor 1510 may be referred to as a transmitter, areceiver, a transmitter/receiver, or a communicator. At least one of thebaseband processor 1520 and the RF processor 1510 may include aplurality of communication modules to support different wireless accesstechnologies, and may include different communication modules configuredto process signals of different frequency bands. For example, thedifferent wireless access technologies may include an LTE network or anNR network, and may include a super high frequency (SHF) band (e.g., 2.5GHz or 5 GHz), and a millimeter wave (e.g., 60 GHz) band.

The storage 1530 may store data such as a default program, anapplication program, and configuration information for the operations ofthe UE 1500 described above with reference to FIGS. 1 to 14. The storage1530 may provide the stored data in response to a request from thecontroller 1540.

The controller 1540 may control overall operations of the UE 1500. Forexample, the controller 1540 may transmit and receive signals throughthe baseband processor 1520 and the RF processor 1510, and may recordand read the data stored in the storage 1530. To do so, the controller1540 may include at least one processor. For example, the controller1540 may include a communication processor (CP) configured to performcommunication control and an application processor (AP) configured tocontrol an upper layer such as an application program.

FIG. 16 is a block diagram of a base station 1600 according to anembodiment.

Referring to FIG. 16, the base station 1600 may include an RF processor1610, a baseband processor 1620, a backhaul communicator 1630, a storage(i.e., memory) 1640, and a controller 1650. However, components of FIG.16 are only an example, and, components of the base station 1600 are notlimited to the above-described example.

The RF processor 1610 may perform transmission and reception of a signalthrough a wireless channel by converting a band of the signal oramplifying the signal, for example. The RF processor 1610 may up-converta baseband signal provided from the baseband processor 1620 to an RFband signal, transmit the RF band signal through an antenna, anddown-convert a RF band signal received through the antenna to a basebandsignal. For example, the RF processor 1610 may include a transmissionfilter, a reception filter, an amplifier, a mixer, an oscillator, a DAC,and an ADC, and while only one antenna is shown in FIG. 16, the basestation 1600 may have a plurality of antennas. The RF processor 1610 mayinclude a plurality of RF chains and may perform beamforming byadjusting a phase and magnitude of each of signals transmitted andreceived via the plurality of antennas or antenna elements. The RFprocessor 1610 may perform a down-MIMO operation by transmitting one ormore layers.

The baseband processor 1620 may perform conversion the baseband signaland a bit string according to a set physical layer specification ofwireless access technology. For example, upon transmitting data, thebaseband processor 1620 may generate complex symbols by encoding andmodulating transmitted bit strings. Upon receiving data, the basebandprocessor 1620 may demodulate and decode the baseband signal providedfrom the RF processor 1610 to reconstruct received bit strings. Forexample, when data is transmitted according to an OFDM scheme, thebaseband processor 1620 may generate complex symbols by encoding andmodulating transmitted bit strings, map the complex symbols tosubcarriers, and configure OFDM symbols by performing an IFFT operationand inserting a CP. Upon receiving data, the baseband processor 1620 maysplit the baseband signal provided from the RF processor 1610 into OFDMsymbol units and restore the signals mapped to the subcarriers byperforming an FFT operation and then reconstruct the received bitstrings by performing demodulation and decoding. The baseband processor1620 and the RF processor 1610 may transmit and receive signals asdescribed above, and may be referred to as a transmitter, a receiver, atransmitter/receiver, or a communicator.

The backhaul communicator 1630 may provide an interface to performcommunication between other nodes in a network.

The storage 1640 may store data such as a default program, anapplication program, and configuration information for the operation ofthe base station 1600 described above with reference to FIGS. 1 to 14.The storage 1640 may store information about a bearer allocated to anaccessed UE, a measurement result reported from the accessed UE, andthat is a reference for determining whether to provide or stop multipleconnections to the UE. The storage 1640 may provide the stored data inresponse to a request from the controller 1650.

The controller 1650 may control overall operations of the base station1600. For example, the controller 1650 may transmit and receive signalsthrough the baseband processor 1620 and the RF processor 1610 or throughthe backhaul communicator 1630, and may record and read the data storedin the storage 1640. To do so, the controller 1650 may include at leastone processor.

According to the embodiments, a UE that is dually accessed to each of aplurality of base stations may effectively perform a data recoveryprocedure when transmitting data via different links connected to eachof the base stations.

Embodiments of the present disclosure may also be embodied as computerreadable code on a non-transitory computer readable recording medium. Anon-transitory computer readable recording medium is any data storagedevice that can store data, which can be thereafter read by a computersystem. Examples of the non-transitory computer readable recordingmedium include read only memory (ROM), random access memory (RAM),CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, anddata transmission through the Internet. The non-transitory computerreadable recording medium can also be distributed over network coupledcomputer systems so that the computer readable code is stored andexecuted in a distributed fashion. In addition, functional programs,code, and code segments for accomplishing the present disclosure can beeasily construed by programmers skilled in the art to which the presentdisclosure pertains.

Embodiments of the present disclosure are only illustrative of thepresent disclosure and are not intended to limit the scope of thepresent disclosure. That is, it is obvious to one of ordinary skill inthe art that other modifications based on the technical idea of thepresent disclosure may be embodied. Each of the above embodiments may becombined with each other as needed. For example, the base station andthe UE may operate by combining parts of the first, second, third, andfourth embodiments of the present disclosure. While the aboveembodiments are presented based on the NR system, other modificationsbased on the technical idea of the embodiments may be embodied in othersystems such as a Frequency division duplex (FDD) LTE system or a timedivision duplex (TDD) LTE system.

While the present disclosure has been shown and described with referenceto embodiments thereof, it will be understood by those skilled in theart that various changes in form and details may be made therein withoutdeparting from the spirit and scope of the present disclosure as definedby the appended claims and their equivalents.

What is claimed is:
 1. A method performed by a terminal in a wirelesscommunication system, the method comprising: receiving a radio resourcecontrol (RRC) message to change or release a secondary cell group (SCG);performing a re-establishment for a radio link control (RLC) entity ofthe terminal based on the RRC message; and performing a packet dataconvergence protocol (PDCP) data recovery based on the RRC message,wherein performing the PDCP data recovery comprises: selectivelyretransmitting a PDCP protocol data unit (PDU) previously submitted tothe re-established RLC entity, for which a successful delivery has notbeen confirmed by the re-established RLC entity.
 2. The method of claim1, further comprising discarding a PDCP service data unit (SDU), forwhich a successful delivery is confirmed by a PDCP status report.
 3. Themethod of claim 2, wherein the PDCP SDU is associated with the PDCP PDU.4. A terminal in a wireless communication system, the terminalcomprising: a transceiver; and at least one processor coupled with thetransceiver and configured to: receive a radio resource control (RRC)message to change or release a secondary cell group (SCG); perform are-establishment for a radio link control (RLC) entity of the terminalbased on the RRC message; and perform a packet data convergence protocol(PDCP) data recovery based on the RRC message, wherein the at least oneprocessor is further configured to: selectively retransmit a PDCPprotocol data unit (PDU) previously submitted to the re-established RLCentity, for which a successful delivery has not been confirmed by there-established RLC entity.
 5. The terminal of claim 4, wherein the atleast one processor is further configured to discard a PDCP service dataunit (SDU), for which a successful delivery is confirmed by a PDCPstatus report.
 6. The terminal of claim 5, wherein the PDCP SDU isassociated with the PDCP PDU.
 7. A method performed by a base station ina wireless communication system, the method comprising: transmitting aradio resource control (RRC) message to change or release a secondarycell group (SCG), wherein a re-establishment for a radio link control(RLC) entity of the terminal, based on the RRC message, is performed,wherein a packet data convergence protocol (PDCP) data recovery, basedon the RRC message, is performed, and wherein a procedure of the PDCPdata recovery comprises selectively retransmitting a PDCP protocol dataunit (PDU) previously submitted to the re-established RLC entity, forwhich a successful delivery has not been confirmed by the re-establishedRLC entity.
 8. The method of claim 7, wherein a PDCP service data unit(SDU), for which a successful delivery is confirmed by a PDCP statusreport, is discarded.
 9. The method of claim 8, wherein the PDCP SDU isassociated with the PDCP PDU.
 10. A base station in a wirelesscommunication system, the base station comprising: a transceiver; and atleast one processor coupled with the transceiver and configured to:transmit a radio resource control (RRC) message to change or release asecondary cell group (SCG), and wherein a re-establishment for a radiolink control (RLC) entity of the terminal, based on the RRC message, isperformed, wherein a packet data convergence protocol (PDCP) datarecovery, based on the RRC message, is performed, and wherein aprocedure of the PDCP data recovery comprises selectively retransmittinga PDCP protocol data unit (PDU) previously submitted to there-established RLC entity, for which a successful delivery has not beenconfirmed by the re-established RLC entity.
 11. The base station ofclaim 10, wherein a PDCP service data unit (SDU), for which a successfuldelivery is confirmed by a PDCP status report, is discarded.
 12. Thebase station of claim 11, wherein the PDCP SDU is associated with thePDCP PDU.